Synthesis and luminescence properties of a Li3BaCaY3(MoO4)8:Er3+ phosphor with a layered scheelite-like structure

Article abstract of DOI:10.1134/S0020168517040082

A Li₃BaCaY₃(MoO₄)₈:Er³⁺ phosphor with a scheelite-like structure (sp. gr. C2/c) has been synthesized and its luminescence properties have been studied. The phosphor has been characterized by X-ray dif

Full text of DOI:10.1134/S0020168517040082

ISSN 0020-1685, Inorganic Materials, 2017, Vol. 53, No. 4, pp. 419–423. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © N.M. Kozhevnikova, 2017, published in Neorganicheskie Materialy, 2017, Vol. 53, No. 4, pp. 417–421.
Synthesis and Luminescence Properties of a Li3BaCaY3(MoO4)8:Er3+
Phosphor with a Layered Scheelite-Like Structure
N. M. Kozhevnikova
Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences,
ul. Sakh’yanovoi 6, Ulan-Ude, 670047 Buryat Republic, Russia
e-mail: nicas@binm.bscnet.ru
Received July 22, 2016
Abstract⎯A Li3BaCaY3(MoO4)8:Er3+ phosphor with a scheelite-like structure (sp. gr. C2/c) has been syn-
thesized and its luminescence properties have been studied. The phosphor has been characterized by X-ray
diffraction, differential thermal analysis, and IR spectroscopy.
Keywords: phosphor, X-ray diffraction analysis
DOI: 10.1134/S0020168517040082
INTRODUCTION
EXPERIMENTAL
The starting chemicals used to study phase rela-
Oxide compounds having layered scheelite-like
crystal structures and activated with rare-earth ions
are potentially attractive as laser materials, phosphors,
and solid electrolytes [1–3]. In recent years, there has
been an intensive search for multifunctional phos-
phors offering bright luminescence under UV exci-
tions
in
the
Li2MoO4–BaMoO4–CaMoO4–
Y2(MoO4)3 system were Li2MoO4, BaMoO4,
CaMoO4, and Y2(MoO4)3 presynthesized by solid-
state reactions in mixtures of the Li2CO3, BaCO3, and
CaCO3 carbonates; molybdenum trioxide, MoO3
tation and anti-Stokes luminescence under IR exci- (analytical grade); and yttrium and erbium oxides
tation. One efficient way of modifying luminescence (99.99+% purity). The Li2CO3, BaCO3, CaCO3, and
properties of materials is by activating them with Er3+ MoO3 compounds were calcined for 10 h at 400°C,
ions, which are capable of absorbing IR radiation and
converting it to visible anti-Stokes luminescence [4–6].
In scheelite-like molybdates and tungstates, one can
stabilize a combination of different cations, atypical
coordination polyhedra, and metastable polymorphs
of three–dimensional structures in the form of layers
[3]. Which structural variety will be realized depends
on the nature of the cations present in the composition
of the compound, their combination, their distribu-
tion over structural sites, and site occupancies. A wide
isomorphism of cations differing in nature leads to
charge disbalance in the structure as a consequence of
specific geometric features in the arrangement of
nearest neighbor polyhedra and to local and coopera-
tive distortions and makes it possible to control the
spectroscopic properties of compounds with layered
structures [3, 7–10].
and the rare-earth oxides were calcined in the tem-
perature range 400–700°C. Li3BaCaY3(MoO4)8 was
synthesized using a reaction mixture of the lithium,
barium, calcium, and yttrium molybdates, 3Li2MoO4 +
2BaMoO4 + 2CaMoO4 + 3Y2(MoO4)3, which was cal-
cined in the temperature range 500–750°C with
repeated intermediate grindings every 20–30 h. The
calcination time at each temperature was 100–120 h.
To obtain different activator concentrations, erbium
oxide (3–5 mol %) was substituted for an equimolar
amount of yttrium molybdate. After firing, the sam-
ples were slowly furnace-cooled. Nonequilibrium
samples were annealed further. Equilibrium was
thought to be reached if the phase composition of the
samples remained unchanged after two sequential
anneals. The synthesis products were identified by X-ray
diffraction on a Bruker D8 Advance diffractometer
(CuKα radiation). X-ray diffraction data were ana-
lyzed using Rentgen software.
The objectives of this work were to synthesize a
Li3BaCaY3(MoO4)8:Er3+ phosphor with a layered
scheelite-like structure and investigate its spectro-
scopic and physicochemical characteristics.
Vibrational spectra of the Li3BaCaY3(MoO4)8:Er3+
phosphor were measured on a Varian 3100 Fourier
419

420
KOZHEVNIKOVA
tion on the twofold axis and have an octahedral oxygen
coordination [3, 9]. The Ba and Ca atoms are in ten
coordination and reside between the layers formed by
the Mo and rare-earth polyhedra.
The X-ray diffraction pattern of Li3Ba-
CaY3(MoO4)8:Er3+ could be indexed in monoclinic
symmetry using the unit-cell parameters of a Li3Ba2-
Gd3(MoO4)8 single crystal: а = 5.171(5) Å, b =
12.532(3) Å, с = 19.114(3) Å, and β = 91.59(1)°. The
Li3BaCaY3(MoO4)8:Er3+ phosphor melts incongru-
ently at 950°C.
The structure of the Li3Ba2Gd3(MoO4)8 single
crystal was solved by the heavy atom method using a
three-dimensional Patterson function. The lighter
atoms O and Li were located using ρ(xyz) electron
density map calculations. Anisotropic structure
refinement, involving gadolinium site occupancy
refinement, converged to R = 0.044. We evaluated the
local valence force balance, taking into account site
occupancies. The data thus obtained are well consistent
with the stoichiometric composition of the Li3Ba2-
1
2
3
10
20
30
40
50
2θ, deg
Fig. 1. X-ray diffraction patterns of (1) Li Ba Y (MoO ) ,
3
2
3
4 8
3+
(2) Li BaCaY (MoO ) , and (3) Li BaCaY (MoO ) :Er
.
3
3
4 8
3
3
4 8
transform IR (FTIR) spectrometer (equipped with a
multiple frustrated total internal reflection auxiliary).
The samples were prepared by pressing with KBr. Dif- Gd3(MoO4)8 compound. The valence balance quality
ferential thermal analysis scans were performed with
an MOM OD-103 thermoanalytical system (Hun-
gary) at a heating rate of 10°C/min (sample weight,
0.3–0.4 g). Luminescence spectra of Li3Ba-
D0 is 2.10% [3, 9].
The measured vibrational frequencies in the long-
wavelength part of the IR spectra of Li3BaCaY3(MoO4)8,
Li3BaCaY3(MoO4)8:Er3+ and Li3Ba2Y3(MoO4)8 are listed
in the table. The effect of the nature of their constitu-
ent cations on the key structural features of
Li3BaCaR3(MoO4)8 was studied by comparing their
IR spectra to those of the Li3Ba2R3(MoO4)8 isostruc-
tural compounds, which are very similar in the spec-
tral region in question (Fig. 2) [11]. In group-theoret-
ical analysis of the vibrational modes of
Li3BaCaR3(MoO4)8, we used analogies with previ-
ously studied double and triple molybdates crystalliz-
ing in a monoclinically distorted layered scheelite
structure (sp. gr. C2/c) [3]. Group-theoretical analysis
CaY3(MoO4)8:Er3+ under UV excitation (λex = 365 nm)
were obtained on an SM 2203 spectrofluorometer
(Solar, Belarus). The excitation was provided by a
DKsSh 150-1M high-pressure xenon arc lamp. In the
IR spectral region, luminescence was excited by an
InGaAs laser diode (λex = 977 nm), and luminescence
spectra were measured on an Ocean Optics QE 65000
spectrometer.
RESULTS AND DISCUSSION
X-ray diffraction characterization showed that the of the vibrational modes of Li3BaCaR3(MoO4)8 sug-
samples annealed at 700°C contained no unreacted
lithium, alkaline-earth, or rare-earth molybdates. The
X-ray diffraction pattern of Li3BaCaY3(MoO4)8:Er3+
was similar in peak positions and intensities to those of
the Li3Ba2R3(MoO4)8 (R = La–Lu) lithium barium
rare-earth molybdates, confirming that they had a
monoclinically distorted scheelite structure (sp. gr.
C2/c, Z = 2) (Fig. 1). The structure of Li3Ba-
CaY3(MoO4)8 contains honeycomb layers of eight-
vertex Y polyhedra. Each layer shares oxygen corners
with Mo tetrahedra on both sides. The lithium atoms
occupy different crystallographic sites. One-third of
gests the following distribution over irreducible repre-
sentations:
Г(Li) = 1Ag + 2Bg + 1Au + 2Bu for the lithium cat-
ions residing on the twofold axes (in octahedra);
Г(М1) = 1Ag + 2Bg + 1Au + 2Bu for the barium, cal-
cium, and rare-earth cations on the twofold axes (M1
site); and
Г(М2) = 3Ag + 3Bg + 3Au + 3Bu for the lithium,
barium, calcium, and rare-earth cations in general
position (M2 site in the eight-vertex polyhedra).
These modes are active in IR absorption spectra
the lithium atoms are located at random on the rare- and should have frequencies under 500 cm–1. The IR
earth site (coordination number CN = 8). The other spectra have large bandwidths, and their bands over-
two-thirds of the lithium atoms occupy a special posi- lap, being a combination of several closely spaced lines
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Vibrational frequencies in the long-wavelength part of the IR absorption spectra of Li3Ba2Y3(MoO4)8,
Li3BaCaR3(MoO4)8, and Li3BaCaY3(MoO4)8:Er3
Wavenumber, cm–1
Assignment
Li3BaCaY3(MoO4)8:Er3+
Li3Ba2Y3(MoO4)8
Li3BaCaY3(MoO4)8
456
417
382
324
290
265
263
232
186
454
416
382
319
287
265
257
227
186
452
417
382
319
288
265
258
230
186
δ(МоО4)
δ(МоО4)
δ(МоО4)
TLi
TLi
TMo + TY
TLi
TLi
TMo + TY
[3, 11]. Moreover, this part of the spectrum should
also contain bands arising from bending, librational,
and translational modes of the MoO4 groups.
The measured spectra of the compounds studied
here have no marked differences in their high-fre-
quency part (660–990 cm–1). The absorption bands in
Doping of the luminescent material Li3Ba-
CaY3(MoO4)8 with erbium ions is an efficient way of
raising the luminescence brightness of IR and visible
phosphors with a scheelite-like structure. Activation
of phosphors with Er3+ ions, which are capable of
absorbing IR light and converting it to visible anti-
this spectral region, due to the valence modes of the Stokes luminescence, makes it possible to raise the
MoO4 groups, are poorly resolved. This is a conse- efficiency of lasers. The spectral composition of
steady-state Li3BaCaY3(MoO4)8:Er3+ luminescence at
quence of the described cation distribution over the
crystallographic sites of the crystals and the associated
statistics of distortions of the tetrahedral anion groups.
In the bending region of the MoO4 groups and the
region of vibrational modes of the cation sublattices
(below 500 cm–1), up to 14 absorption bands were
detected in the spectrum of Li3BaCaY3(MoO4)8:Er3+.
At the same time, in this region the spectra of the
compounds under consideration are very similar to
those of Li3Ba2R3(MoO4)8. This indicates that the
vibrational modes of the cation sublattices are similar
to each other and to the bending modes of the MoO4
tetrahedra.
λex = 365 nm is determined by bands corresponding to
optical transitions of the Er3+ ion. As the Er3+ content is
raised to 5 mol %, several groups of erbium-related nar-
row emission bands emerge in the green (535–610 nm)
1
2
3
The frequencies of the external vibrational modes
of the MoO4 groups and the vibrational modes of the
heavy cation sublattices are roughly the same as in
Li3Ba2Y3(MoO4)8.
The librational modes of the MoO4 groups are
responsible for the bands in the range 156–164 cm–1,
and vibrations of atoms in the Sr and Ba sublattices are
responsible for the bands at 102–103 and 90–91 cm–1,
respectively. The translational modes of the molybde-
num are intermixed with vibrations of atoms in the
rare-earth sublattice and have frequencies of 265 and
186 cm–1. In the long-wavelength part of the spectra of
the compounds under study, we detected two bands, at
227–232 and 257–263 cm–1, attributable to vibrations
of the lithium cations [5].
200
400
600
800
1000
Wavenumber, cm–1
Fig. 2. IR spectra of (1) Li Ba Y (MoO ) , (2) Li Ba-
CaY (MoO ) , and (3) Li BaCaY (MoO ) :Er
3
2
3
4 8
3
3+
.
3
4 8
3
3
4 8
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422
KOZHEVNIKOVA
977 nm can be understood as follows: According to a
(а)
known energy diagram [5], after a sequential, two-step
1200
1000
4S3/2 → 4I15/2
excitation of the Er3+ ion to the 4F7/2 level, nonradia-
tive relaxation processes lead to the filling of the 4Н11/2
,
4S3/2, 4F9/2, and 4I9/2 excited-state levels, giving rise to
anti-Stokes luminescence in the range 530–850 nm.
Figure 3 shows the anti-Stokes luminescence spec-
trum between 450 and 850 nm. There are four strong
bands, located around 530, 550, 660, and 800 nm.
800
600
400
200
The 530- and 550-nm bands correspond to the
3+ 2
4
Er
H
11/2
→ 4I15/2 and S3/2 → 4I15/2 transitions. The
660-nm emission is due to the 4F9/2 → 4I15/2 transition,
4
and the 800-nm emission, to the I9/2 → 4I15/2 transi-
0
350 450
550
650
750
850
tion. The weak blue band at 490 nm arises from the
Wavelength, nm
4F7/2 → 4I15/2 transition. A characteristic feature of the
Er3+ ion is a narrow luminescence bandwidth. The
anti-Stokes luminescence excitation process typically
involves a sequential absorption of two IR photons of
the same energy, which leads to the following
(b)
12000
10000
8000
6000
4000
2000
4S3/2
4
4
sequence of electronic transitions: I15/2 → I11/2 and
4I11/2 → 4F7/2. Transitions from high energy levels may
result in visible luminescence. The shape of the spec-
tra can be understood in terms of the effect of the host
crystal lattice on the Stark structure of the ground and
excited-state levels of the activator ion.
2H11/2
4I9/2
CONCLUSIONS
4F9/2
4F7/2
A Li3BaCaY3(MoO4)8:Er3+ phosphor with a lay-
ered scheelite-like structure (sp. gr. C2/c) has been
prepared by solid-state reaction. The luminescence
data obtained in this study demonstrates that, as the
Er3+ content is raised to 5 mol %, several groups of
erbium-related narrow emission bands emerge in the
green (535–610 nm) and red (660–705 nm) regions of
the visible spectrum.
0
500
600
700
800
900
Wavelength, nm
Fig. 3. Luminescence spectra of Li BaCaY
-
2.85
3
Er (MoO ) at λ = (a) 365 and (b) 977 nm.
0.15
4 8
ex
and red (660–705 nm) regions of the visible spectrum.
The emission bands in the range 590–610 nm, corre-
sponding to the 4S3/2 → 4I15/2 transition, have the sim-
plest and best-resolved structure. To the left and right
of these green bands, we observe weaker emission
bands, in the ranges 535–538 and 556–560 nm, due to
the H11/2 → I15/2 and H9/2 → I13/2 transitions,
respectively. The bands in the red spectral region are
due to the 4F9/2 → 4I15/2 and 2H9/2 → 4I11/2 transitions,
whose spectrum extends from 660 to 705 nm and has a
more complex structure, because the 4F9/2 excited state
is split into five multiplets.
Li3BaCaY3(MoO4)8:Er3+ possesses efficient anti-
Stokes luminescence in the visible range under IR
excitation. The origin of the bands observed in its anti-
Stokes luminescence spectra under excitation at λex =
977 nm can be understood in terms of the filling of the
4Н11/2, 4S3/2, 4F9/2, and 4I9/2 excited-state levels, giving rise
to anti-Stokes luminescence in the range 530–850 nm.
The Li3BaCaY3(MoO4)8:Er3+ phosphor may find
application in lasers, converters of IR radiation to vis-
ible light, color displays, biomedical diagnostics, and
optical communication systems.
2
4
2
4
Li3BaCaY2.85Er0.15(MoO4)8 possesses efficient anti-
Stokes luminescence in the visible range under IR
excitation. The origin of the bands observed in its anti-
Stokes luminescence spectra under excitation at λex =
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Translated by O. Tsarev
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